1. Field of the Invention
This invention is concerned with the manufacture of high-quality lubricating oils, and in particular is concerned with the catalytic dewaxing and hydrotreating of waxy distillate lubricating oils.
2. Discussion of Prior Art
The refining of petroleum crude oils to obtain lubricant stocks is based primarily on a series of steps including distillation, solvent refining and dewaxing.
For the preparation of a high grade distillate lubricating oil stock, the usual practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. to 1050.degree. F. After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g, furfural, phenol, sulfolane, n-methylpyrrollidone or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example, by admixing it with a solvent, such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes which are then separated from the raffinate. Sufficient quantities of wax are removed to provide the desired pour point for the raffinate.
Other processes, such as hydrofinishing or clay percolation, may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
Viscosity Index (V.I.) is a quality parameter of considerable importance for distillate lubricating oils used in automotive engines and aircraft engines which are subject to wide variations in temperature. This index indicates the degree of change of viscosity with temperature. For example, a high viscosity index, e.g., 100 or greater, indicates an oil that does not tend to become viscous at low temperatures or become thin at high temperatures. Measurement of the Saybolt Universal Viscosity of an oil at 100.degree. and 210.degree. F., and referral to standard industry-wide correlations, provides a measure of the V.I. of the oil. For purposes of the present invention, whenever V.I. is referred to, it designates the V.I. as noted in the Viscosity Index tabulations of the ASTM (D567), published by ASTM, 1916 Race Street, Philadelphia, PA, or equivalent thereof.
To prepare high V.I. automotive and aircraft oils the refiner usually selects a crude oil relatively rich in paraffinic hydrocarbons, since experience has shown that crudes poor in paraffins, such as those commonly termed "naphthene-base" crudes, yield little or no refined stock having a V.I. above about 40. Suitable stocks for high V.I. oils, however, also contain substantial quantities of waxes which result in solvent-refined lubricating oil stocks of high pour point. Thus, in general, the refining of crude oil to prepare acceptable high V.I. distillate stocks ordinarily includes dewaxing to reduce the pour point.
In recent years catalytic techniques have become available for dewaxing petroleum stocks. Although some attention has been directed to treating gas oils and manufacturing specialty oils, of primary interest has been and is the catalytic dewaxing and subsequent treatment of lube oil stocks. Processes relating to the dewaxing of gas oils and specialty oils are described in U.S. Pat. Nos. 3,894,938 and 4,137,148, the contents of both of which are incorporated herein by reference.
U.S. Pat. No. 3,894,938 discloses a catalytic dewaxing process in which high-pour-point, high-sulfur gas oils having a boiling range of about 400.degree. F. to 900.degree. F. are first contacted, in the presence or absence of added hydrogen, with a ZSM-5 type zeolite hydrodewaxing catalyst which may contain a hydrogenation/dehydrogenation component. The effluent therefrom is subsequently desulfurized and denitrogenated by contacting it with a cobalt-molybdenum-alumina catalyst.
U.S. Pat. No. 4,137,148 discloses a process wherein specialty oils of low pour point and excellent stability are produced from waxy crude distillates having a boiling range of 450.degree. F. to 1050.degree. F. by solvent refining, catalytic dewaxing over a zeolite catalyst, such as ZSM-5, and hydrotreating (hydrofinishing). The catalytic dewaxing reaction produces olefins which would impair properties of the dewaxed oil product if they were retained in the product. The olefins are saturated by hydrogenation in the hydrotreater, as confined by chemical analysis of the hydrotreated product for bromine number. Low bromine numbers are an indication of a satisfactory level of saturation. The hydrotreating step comprises cascading the effluent from the catalytic dewaxing step into a hydrotreating reactor of the type now generally employed for the finishing of lubricating oil stocks. Many of the known hydrotreating catalysts consisting of a hydrogenation component on a non-acidic support can be employed, for example, cobalt-molybdate or nickel-molybdate or molybdenum oxide, on an alumina support. Subsequent to this treatment, the effluent of the hydrotreater is topped by distillation to meet flash and firepoint specifications.
Techniques for dewaxing and subsequent treating of lubricating oil stocks are exemplified in U.S. Pat. Nos. 3,755,138 and 4,222,855, the contents of which are incorporated herein by reference.
U.S. Pat. No. 3,755,138 discloses a process wherein a lube oil stock boiling between 650.degree. F. and 1100.degree. F. is subjected to mild solvent dewaxing and subsequently to hydrodewaxing. The hydrodewaxing step constitutes contacting the lube oil stock with a crystalline aluminosilicate of the ZSM-5 type which contains a metal hydrogenating component in the presence of added hydrogen. In U.S. Pat. No. 4,222,855 lube oil stocks boiling between 600.degree. F. and 1050.degree. F. are catalytically dewaxed by contacting them with a crystalline aluminosilicate having particularly characterized pore openings, such as ZSM-23 and ZSM-35.
U.S. Pat. No. 3,668,113, the entire contents of which are incorporated herein by reference, discloses a process in which petroleum fractions, such as gas oil and wax distillate fractions, are first passed over a catalyst comprising a crystalline mordenite of reduced alkali metal content and a metal hydrogenating component to remove wax. The reaction product is then passed over a catalyst comprising a refractory inorganic oxide support and a hydrogenating component selected from metals and compounds thereof of Groups VI and VIII of the Periodic Table to remove sulfur.
U.S. Pat. No., Re. 28,398, the entire contents of which are incorporated herein by reference, discloses a catalytic process for dewaxing of oils by converting straight-chain and slightly branched-chain hydrocarbons by contacting the oils with a crystalline zeolite having specified molecular sieving properties, e.g., ZSM-5 type and ZSM-8 type zeolites, optionally containing a hydrogenation/dehydrogenation component.
It has also been proposed to combine, in series, severe lube hydrotreating (LHDT) and catalytic lube dewaxing processes (LDW) in order to improve resistance of products to light, e.g., see Garwood et al., U.S. Pat. No. 4,283,271 and Garwood et al., U.S. Pat. No. 4,283,272. However, the effluent of the LHDT process is essentially free from sulfur, which may adversely affect processing thereof after the LDW process.